Molding compositions based on ethylene polymers for producing articles such as caps and closures of all types, engineering parts, and fittings for automobiles, such as guiders, handles, roll-over-valves and fuel tank module parts, by means of injection molding are known. Such articles have a number of requirements, for example they have to retain their dimensions and shape, i.e. not to shrink (low shrinkage) and to show low warpage, after the injection molding process, i.e. on cooling. The injection molding process is generally made easier when the polyethylene molding compositions readily flow in the melt (good flowability). Low shrinkage and shape stability are important properties of polyethylenes from which, for example, accurately fitting engineering parts are to be produced.
Ethylene polymers with properties meeting the above mentioned requirements have hitherto been the so-called multimodal products which comprise at least one relatively low-molecular-weight polymer component and at least one relatively high-molecular-weight polymer component within the polymer. Particularly good properties are obtained when the comonomer content of the relatively low-molecular-weight polymer component is minimized and the comonomer content of the relatively high-molecular-weight polymer component is maximized.
The simplest method of achieving this is to produce the at least two components separately and mix them with one another. As an alternative, the polymer components can be produced in series or in parallel in-situ. In series, this is achieved in a cascade process by preparing one of the polymer components in a first stage and preparing the second component in the subsequent stage. For this, substantial use is made of Ziegler catalysts, which have good hydrogen-controllability and therefore make it easy to adjust the molar mass within the stages. Chromium catalysts are substantially unsuitable for this purpose because they have insufficient hydrogen-controllability. Finally, in relatively recent times attempts have been made to produce the relatively high-molecular-weight and relatively low-molecular-weight components by using what are known as hybrid or mixed catalysts. These generally comprise two or more catalyst components, which can produce the relatively high-molecular-weight and relatively low-molecular-weight polymer components in parallel.
Ethylene polymers prepared using chromium catalysts are particularly suitable for production of blown films, and for blow molding. However, products prepared with the aid of a chromium catalyst have unsuitable comonomer distribution because most of the comonomer is incorporated within the low-molecular-weight fraction of the polymer. The result of this is major restrictions on product properties, in particular the stiffness, impact strength, and environmental stress cracking resistance (ESCR) relation.